The following relates generally to medical imaging and radiation therapy. It finds particular application in conjunction with real-time imaging of anatomical information using ultrasound and delivery of radiation therapy, and will be described with particular reference thereto. However, it will be understood that it also finds application in other usage scenarios and is not necessarily limited to the aforementioned application.
Radiation therapy is a procedure which uses radiation to treat a patient, often to kill or destroy harmful tissues, e.g. tumors. Radiation therapy is applied to the tissues of a subject to minimize radiation to healthy tissues. Healthy tissues can include organs at risk (OARs) such as a heart, liver, kidneys, urethra, rectum, bladder, etc. which may be in close proximity to or include unhealthy tissue. Precise placement of radiation during delivery is important to preserve functioning organs while destroying the unhealthy or diseased tissue. Two conventional radiation therapy techniques include brachytherapy and external beam radiation therapy (EBRT). Anatomical images for planning of radiation therapy typically use X-ray computed tomography (CT) which provides detailed anatomical information, but uses x-ray radiation to obtain the anatomical images.
Brachytherapy typically uses low dose radiation point sources or seeds which are implanted in unhealthy tissues of the subject. The seeds are distributed or dropped through a needle inserted into the unhealthy tissue to provide radiation to surrounding unhealthy tissue. The seeds provide a localized source of radiation. Typically, 50-100 seeds can be implanted into the prostate. Brachytherapy is commonly used for treatment of breast, cervical, prostate, and skin cancers. Precise placement or delivery of the seeds relative to the unhealthy and OARs determines the dose of radiation delivered to the tumor and the OARs. Multiple seeds are distributed in the unhealthy tissue to provide coverage. The coverage is based on the location of each dropped seed and the amount of radiation emitted by each point source.
EBRT delivers external linear beams of radiation through the body to the target tissues, using a EBRT device such as a linear accelerator (LINAC). Typically, the beams can be shaped and aimed from different directions to help avoid striking OARs. Knowing the precise position of the target and OARs relative to the radiation beam when the external radiation beam is active determines the dose delivery to both the tumor and any healthy tissues in the path of the radiation beam.
In the delivery of radiation therapy, the precise placement of the radiation relative to the anatomical locations of unhealthy and healthy tissue affects the outcome, particularly the sufficiency of the dose to kill the unhealthy tissue and the side effects of the dose to the healthy tissue. During the delivery of radiation, the location of the healthy and unhealthy tissue is subject to movement of the patient. The movement can include rigid movement, non-rigid movement, deformation of organs, and repetitive movement due to respiration and/or cardiac cycles. One challenge is matching the location of the therapeutic radiation during delivery to the anatomical location of the target and OARs. In other words, one challenge is to register the dosimetric information and anatomical information in a common coordinate system during delivery. A second challenge is measuring the motion accurately in substantially real time such that adjustments can be made during the delivery to correct for position and/or motion.
For example, in brachytherapy, anatomical information can be obtained from ultrasound, but matching the location of the dropped seeds with the target and OARs can be problematic. Ultrasound (US) can provide continuous real-time anatomical images during delivery of therapeutic radiation without using x-ray radiation. However, visibility of seeds can be obscured or confused with shadowing or other artifacts, which leads to poor sensitivity and specificity. Poor placement of seeds can lead to cold spots or areas where the target is not receiving the proper radiation dose or has spots where OARs receive more than prescribed.
One approach has been to take intermediate CT images which contrast the seeds with less visibility to organ boundaries in high contrast, but which involves moving the patient from a brachytherapy operative configuration to a CT imaging configuration and returning the patient back to the operative environment to make adjustments for seed placement. The two patient movements are apt to create registration errors between the CT and brachytherapy coordinate systems. The use of CT also adds additional x-ray dose to the patient.
Another approach is the use of intermittent fluoroscopic images which images the seeds with good contrast and reduces the x-ray imaging dose to the patient compared to CT. However, fluoroscopic images provide less anatomic contrast and are taken intermittently, which makes matching of seed locations to the anatomic locations difficult.
In EBRT, beams of radiation are directed through the subject. Typically external markers placed on the patient's skin are used to register the patient to the coordinate system of the EBRT delivery device and the coordinate system of high resolution planning images. However, the external marks provide poor internal anatomic information. Due to tissue pliability, registration errors between the external markers and the target and OARs can occur. Without precise anatomic information in a common coordinate system with the external beam coordinate system during delivery of radiation, incomplete dose coverage of the tumor and significant damage to OARs can result. One approach to protecting OARs is to exclude radiation delivery from a margin around OARs large enough to include the entire range of motion. One approach to assuring delivery of the prescribed dose is to irradiate a margin around the target such that the target is in the beam over the range of motion. Due to proximity of the target and OARs, assuring full dose to the target and minimal dose to OARs are often conflicting goals.
Another approach to identifying the precise location of the organ boundaries and/or motion is the use of electromagnetic (EM) tracking technology. However, EM tracking is sensitive to external distortions such as metallic equipment, prostheses in patients, pacemakers, etc., and depends on positioning of the EM field generator.